Method and apparatus for measuring and controlling the surface characteristics of sheet materials such as paper

Abstract

An on-line laser triangulation position sensing system provides measurements of the surface features of a moving sheet such as paper. The measurements are correlatable with measurements made with standard laboratory surface smoothness testers and provide predictions of the printability of paper during its fabrication. The sensing system includes a segmented balance detector having a high frequency response. The sensing system further includes signal processing circuitry comprising a plurality of channels, each channel including a filter. The various filters have different cutoff frequencies for passing different frequency spectra representing ranges of surface feature scale sizes. The cutoff frequency of each filter is varied in response to variations in sheet speed. The sheet sensing system further includes a sheet stabilizer to minimize flutter in the measurement region and a standardizing member permitting performance verification of the sensor system at at least one scale size spectrum. The sensing system may also be provided with a sheet compressor to compress the sheet in the measurement region so as to simulate the pressures applied to sheet samples by laboratory air leak smoothness testers. The output of the position sensing system may be used in conjunction with various kinds of papermaking machines to control one or more of the many papermaking process parameters that determine the smoothness or texture of the final product. The sensing system can be used to measure tissue crepe and such measurement can be utilized to control various crepe-determining stages in a tissue making machine and to monitor the condition of the creping or doctor blade in such a machine.

What is claimed is: 
     
       1. An apparatus for the continuous, on-line measurement of a property of a surface of a moving sheet, said property being characterized by surface features having various scale sizes and heights, the apparatus comprising: a laser light source;   means for focusing incident light from the laser source along an optical path intercepting the surface of the moving sheet to illuminate a light spot on said surface;   means for collecting light scattered at a non-specular angle from the illuminated spot;   a photosensitive detector responsive to a spectrum of frequencies produced by the various surface feature scale sizes, the detector having an output;   means for focusing the collected scattered light on said photosensitive detector, the output of the detector providing an output signal representing variations of the height position of the light spot on the surface of the moving sheet; and   a plurality of channels each including a filter responsive to the output signal of the detector, said filters having different cut-off frequencies for passing different frequency spectra representing different ranges of surface features scale sizes.   
     
     
       2. An apparatus, as defined in claim 1, including: means for monitoring the speed of the moving sheet and providing an output indicative of said speed;   and in which: the cut-off frequency of each of the filters is varied in response to variations in the output of the sheet speed monitoring means.     
     
     
       3. An apparatus, as defined in claim 1, in which: the photosensitive detector comprises a balance detector generating a pair of signals, the detector output signal being proportional to the difference between the signals of said pair of signals.   
     
     
       4. An apparatus, as defined in claim 3, including: means between the detector and the at least one channel for processing the pair of detector signals, said signal processing means providing an output that is a function of the ratio of the difference between the pair of signals to the sum of the pair of signals.   
     
     
       5. An apparatus, as defined in claim 3, in which: the balance detector comprises a pair of side-by-side photosensitive cells separated by a small linear gap, and in which the light focused on the balance detector bridges the gap so as to illuminate to a greater or a lesser extent the detector cells substantially in accordance with the height position of the light spot illuminating the surface of the moving sheet.   
     
     
       6. An apparatus, as defined in claim 5, in which: the linear gap separating the balance detector cells is disposed at an angle to the direction of movement of the light spot incident on the detector to extend the range of the detector.   
     
     
       7. An apparatus, as defined in claim 1, in which: each filter comprises a high pass filter.   
     
     
       8. An apparatus, as defined in claim 7, including: means for monitoring the speed of the moving sheet and for providing an output indicative of said speed;   and in which: said high pass filter comprises an RC filter having an input connected to the output of the photosensitive detector, a variable resistor, and an output across the variable resistor, and including means for controlling the value of the resistor in response to the output of the speed monitoring means.     
     
     
       9. An apparatus, as defined in claim 8, in which: said resistor value controlling means comprises an XY multiplier circuit having a first input connected to the output of the sheet speed monitoring means and a second input connected to the output of the filter.   
     
     
       10. An apparatus, as defined in claim 1, in which: each filter comprises a bandpass filter.   
     
     
       11. An apparatus, as defined in claim 1, which includes: means defining a reference surface along which the surface of the sheet is adapted to move in close proximity, said reference surface lying substantially in a plane at which the incident light focusing means focuses the light spot illuminating the surface of the sheet; and   a sheet stabilizer for maintaining the surface of the moving sheet in close proximity to the reference surface.   
     
     
       12. An apparatus, as defined in claim 1, which includes: means defining a reference surface along which the surface of the sheet is adapted to move in close proximity, said reference surface lying in a plane at which the incident light focusing means focuses the light spot illuminating the surface of the sheet;   a standardizing member having an optical standardizing surface; and   means for moving said standardizing member into an off-sheet standardizing position in which the optical standardizing surface is positioned substantially in the plane of the reference surface.   
     
     
       13. An apparatus, as defined in claim 12, further including: means for oscillating the position of the standardizing member in a direction along the optical axis of the incident light at at least one predetermined frequency and at least one predetermined amplitude representing, respectively, at least one surface feature scale size and at least one surface feature height variation.   
     
     
       14. An apparatus, as defined in claim 1, in which: the filter of each channel has an output; and   each channel includes an RMS AC to DC converter connected to the output of the filter associated with that channel, each said RMS AC to DC converter providing a channel output.   
     
     
       15. An apparatus, as defined in claim 14, in which: each filter is a high pass filter having a predetermined cutoff frequency;   the apparatus further including:   an arithmetic unit connectable to the outputs of two of the channels, the arithmetic unit providing an output indicative of the height position variations within the scale size range corresponding to the cutoff frequencies of the filters in said two channels.   
     
     
       16. An apparatus, as defined in claim 1, in which: the light spot produced on the surface of the sheet by the incident light focusing means has a width no greater than about 20 micrometers.   
     
     
       17. An apparatus, as defined in claim 1, in which: the plurality of channels span a corresponding plurality of contiguous scale size ranges.   
     
     
       18. An apparatus, as defined in claim 1, in which: each filter comprises a low pass filter.   
     
     
       19. An apparatus, as defined in claim 1, in which: the lowest cut-off frequency is above the frequency of low frequency phenomena such as sheet flutter.   
     
     
       20. An apparatus, as defined in claim 1, in which: the surface property comprises creping.   
     
     
       21. An apparatus for the continuous, on-line measurement of the topographical features of a surface of a moving sheet, said features having various scale sizes and heights, the apparatus comprising: a laser triangulation position sensor for illuminating a light spot on the surface of the moving sheet and including a detector responsive to an image of the light spot reflected from the surface substantially along an optical axis of reflection, the detector comprising a pair of cells separated by a linear gap, the cells defining an active photosensitive area intercepting the reflection axis so that the image received by the detector bridges the gap to illuminate to a greater or a lesser extent the detector cells thereby sensing position deviations of the image substantially in accordance with variations in the height position of the light spot illuminating the surface of the moving sheet, the image being displaceable along an axis on the active area intercepting the linear gap, the detector having an output for providing a signal representing variations of the height position of the light spot illuminating the surface of the moving sheet, the linear gap being disposed at an angle lying between the image displacement axis and an axis orthogonal thereto, the angular orientation of the linear gap extending the measurement range of the detector.   
     
     
       22. An apparatus, as defined in claim 21, in which the sensor includes: a source of laser light;   means for focusing light from the laser source along an optical path of incident light substantially perpendicular to the surface of the moving sheet to illuminate the light spot on said surface; and   means for collecting scattered light reflected at a non-specular angle from the illuminated light spot and focusing the collected light on the active area of the detector substantially along the reflection axis.   
     
     
       23. An apparatus, as defined in claim 21, in which: the detector is responsive to a spectrum of frequencies produced by surface feature scale sizes of at least 20 micrometers.   
     
     
       24. An apparatus, as defined in claim 21, in which: each cell of the detector is adapted to generate an electrical signal indicative of the extent of illumination of that cell by the reflected image, the detector output signal being substantially proportional to the difference between the signals generated by the cells of the detector.   
     
     
       25. An apparatus, as defined in claim 21, in which: each cell of the detector is adapted to generate an electrical signal indicative of the extent of illumination of that cell by the reflected image, the detector output signal being a function of the ratio of the difference between the signals generated by the cells to the sum of said signals.   
     
     
       26. An apparatus, as defined in claim 21, in which: the light spot illuminated on the surface of the moving sheet has a width no greater than about 20 micrometers.   
     
     
       27. An apparatus, as defined in claim 21, which includes: at least one channel having a filter responsive to the output signal of the detector for filtering out variations in the detector output signal resulting from low frequency phenomena such as sheet flutter and passing frequencies in the detector output signal representing a range of surface feature scale sizes.   
     
     
       28. An apparatus, as defined in claim 27, including: a plurality of channels each including a filter, said filters having different cut-off frequencies for passing different frequency spectra representing different ranges of surface feature scale sizes.   
     
     
       29. An apparatus, as defined in claim 28, including: means for monitoring the speed of the moving sheet and providing an output indicative of said speed;   and in which: the cut-off frequency of each of the filters is varied in response to variations in the output of the sheet speed monitoring means.     
     
     
       30. An apparatus, as defined in claim 29, in which: each filter comprises a high pass filter.   
     
     
       31. An apparatus, as defined in claim 29, in which: each filter comprises a bandpass filter.   
     
     
       32. An apparatus, as defined in claim 28, in which: the filter of each channel has an output; and   each channel includes an AC to DC converter connected to the output of the filter associated with that channel, the AC to DC converter of each channel generating a DC output signal indicative of the true RMS value of the filtered signal.   
     
     
       33. A method for measuring the surface features of a moving sheet, said surface features having various scale sizes and heights, said method comprising the steps of: focusing light from a source along an optical path intercepting the surface of the moving sheet to illuminate a light spot on said surface;   collecting light scattered at a non-specular angle from the illuminated spot;   providing, in response to the collected light, a signal representing variations in the heights of said surface features and having a frequency spectrum produced by the various scale sizes of said surface features; and   processing the signal to produce a plurality of channels covering a total frequency range selected according to the total range of surface feature scale sizes of interest, each channel covering a surface feature scale size range that is different than the surface feature scale size ranges of the remaining channels.   
     
     
       34. A method, as defined in claim 33, further including the steps of: providing a filtering cutoff frequency for each channel;   monitoring the speed of the moving sheet; and   controlling the cutoff frequency of each channel in response to variations in the speed of said sheet.   
     
     
       35. A method, as defined in claim 33, further including the steps of: filtering the signal to produce a plurality of filtered outputs, each of said filtered outputs being associated with one of the plurality of channels, wherein each filtered output is derived from said signal by filtering out all frequency components of the first signal above a predetermined frequency, and wherein the predetermined frequency is different for each filtered output.   
     
     
       36. A method, as defined in claim 35, further including the step of: producing a separate DC output from each filtered output, each DC output being indicative of the true root mean square value of a filtered output.   
     
     
       37. A method, as defined in claim 33, further including the steps of: storing values of said signal during predetermined, successive data acquisition time intervals, each said time interval having a beginning;   providing successive values of the data acquired during successive time intervals;   measuring the speed of the sheet;   adjusting the beginning of said successive time intervals in response to the measured speed of the sheet;   obtaining the difference between the values of the data acquired during successive time intervals; and   averaging the differences of the values acquired during successive time intervals.   
     
     
       38. A method, as defined in claim 33, in which: the plurality of channels span a corresponding plurality of contiguous scale size ranges.   
     
     
       39. A method, as defined in claim 33, further including the steps of: monitoring the speed of the moving sheet; and   adjusting each channel in response to speed of the sheet to hold substantially constant the scale size range of each channel.   
     
     
       40. A method, as defined in claim 33, in which: the moving sheet comprises paper and the surface features comprise creping.   
     
     
       41. An apparatus for the continuous on-line measurement of a characteristic of a surface of a moving paper sheet having opposed surfaces, the apparatus comprising: a laser triangulation position sensor disposed adjacent one of the sheet surfaces for illuminating a light spot on said sheet surface, the sensor including a detector having an output for providing a signal representing variations of the height position of the light spot illuminating the surface of the moving sheet, the sensor further including a bottom, planar surface; and   a backing platform positioned adjacent the other of said sheet surfaces, the backing platform including means for biasing the region of the sheet illuminated by the light spot toward the bottom surface of the sensor to compress said region of said moving paper sheet.   
     
     
       42. An apparatus, as defined in claim 41, in which: the biasing means comprises an inflatable bellows having opposed end portions, one of the end portions being attached to the platform, the other of the end portions being positioned to urge the sheet toward the bottom surface of the sensor.   
     
     
       43. An apparatus, as defined in claim 42, including: a stabilizer arm connecting the other end of the bellows to the platform, the stabilizer arm resisting the tendency of the one end of the bellows to be carried along by said moving sheet.   
     
     
       44. An apparatus for the continuous, on-line measurement of a property of a surface of a moving sheet, said property being characterized by surface features having various scale sizes and heights, the apparatus comprising: a laser light source;   means for focusing incident light from the laser source along an optical path intercepting the surface of the moving sheet to illuminate a light spot on said surface;   means for collecting light scattered at a non-specular angle from the illuminated spot;   a photosensitive detector responsive to a spectrum of frequencies produced by the various surface feature scale sizes, the detector having an output;   means for focusing the collected scattered light on said photosensitive detector, the output of the detector providing an output signal representing variations of the height position of the light spot on the surface of the moving sheet;   at least one channel including a filter responsive to the output signal of the detector for filtering out low frequency variations in the detector output signal and passing frequencies in the detector output signal representing a range of surface feature scale sizes;   means defining a reference surface along which the surface of the sheet is adapted to move in close proximity, said reference surface lying in a plane at which the incident light focusing means focuses the light spot illuminating the surface of the sheet;   a standardizing member having an optical standardizing surface; and   means for moving said standardizing member into an off-sheet standardizing position in which the optical standardizing surface is positioned substantially in the plane of the reference surface.   
     
     
       45. An apparatus, as defined in claim 44, further including: means for oscillating the position of the standardizing member in a direction along the optical axis of the incident light at least one predetermined frequency and at least one predetermined amplitude representing, respectively, at least one surface feature scale size and at least one surface feature height variation.